1. Field of the Invention
This invention relates generally to transmissions and particularly to a range selector valve for heavy duty transmissions.
2. Brief Description of the Background Art
Conventional, commercially available range selector valves for use in transmissions of heavy equipment are susceptible to the possibility that more than one gear will be engaged at any one time due to a hydraulic or electrical malfunction. For example, an inappropriate combination of solenoids may be electrically energized as a result of an electrical malfunction.
In the range selector valve now marketed by Twin Disc Inc., of Racine, Wis., a latch plate is used to provide a fail in gear feature. The Twin Disc arrangement, as implemented, for example, on their TD-61-2607 transmission, uses a plurality of essentially parallel valves to select the transmission splitter and range settings. With this arrangement an accidental actuation could cause multiple settings to be achieved, resulting in severe consequences.
The commercially available Detroit Diesel Allison 5000, 6000 series transmissions employ a cascade oil flow circuit to eliminate the possibility of accidental multiple gear engagements. When valve A is activated, the oil flow is diverted to the clutch A. Since all of the flow is diverted, no clutch down stream can be pressurized. If a clutch upstream from the clutch A is pressurized, all flow will be diverted to that clutch and clutch A will be deactivated.
A number of selector valves are shown in the prior art. Representative of these efforts are U.S. Pat. Nos. 4,135,610, 4,046,160, 3,990,553, 3,944,035, 3,941,007, 3,762,518, 3,468,194, and 3,274,858.
It would be highly desirable to provide a transmission with a range selector valve that eliminates the possibility of simultaneous multiple gear engagements. Moreover, it would be highly desirable to provide such an arrangement that is simpler in design than those previously known.